1. Field of the Invention
The present invention relates to an imaging lens system and an imaging apparatus.
2. Description of the Related Art
In recent years, digital still cameras and digital video cameras have become common as an imaging apparatus in which a solid-state image sensing device such as CCD, CMOS, or the like is used. In particular, such digital still cameras have been widely used instead of silver salt cameras.
Regarding a solid-state image sensing device used in such an imaging apparatus, a large number of pixels have been required. Accordingly, a high optical performance has been desired for the imaging lens. Furthermore, a compact-sized imaging apparatus having a design for portability has also been developed, and an imaging apparatus having both a high performance and a compact size has been greatly desired in the market. Accordingly, an imaging lens which not only has high-performance but is also compact-sized is desired as well.
Furthermore, speed of photographing has steadily increased, and a brighter lens as the imaging lens is desired.
In addition, in a case where a photographed image is received by the solid-state image sensing device, it is preferable that a light beam incident on a light receiving surface of the solid-state image sensing device enter the light receiving surface at a small incident angle. That is, an incident angle of an off-axial light beam is restricted in the imaging apparatus in which the solid-state image sensing device is used, and so-called shading occurs when the incident angle becomes large.
That is, a high telecentric performance in an image side is required for the imaging lens.
Furthermore, in terms of a compact size, it is preferable that the imaging lens be constituted of a small number of lenses.
In a case of a single-focus imaging lens used for a hand-held terminal and the like, which has been widely used in recent years, since a small-sized solid-state image sensing device is used, a configuration of the imaging lens having two or three lenses is effective. However, in a case where the number of pixels is 7 million or more, a large-sized solid-state image sensing device is required, so that it is difficult to sufficiently correct aberrations in a case of a small number of lenses, and a desired performance is difficult to be achieved.
If the number of lenses is increased in order to deal with the above, an entire optical length and size of a lens system are also increased, and a compact-sized lens system is difficult to be achieved. If an increase in the size of the lens system caused by an increase in the number of the lenses is to be suppressed by reducing a size of each lens, a refractive power of the small-sized lens is increased, so that influence due to manufacturing errors or assembly errors is increased, productivity worsens, and production cost increases.
Conventionally, Japanese Patent No. 3076098, Japanese Patent No. 3595897, Japanese Patent No. 3281583, and Japanese Patent No. 4248956 are known as documents disclosing an imaging lens including three lens groups having four lenses, which has the same lens-group configuration as that of an imaging lens according to the present invention.
An imaging lens disclosed in Japanese Patent No. 3076098 does not necessarily suffice for a performance which has been desired in recent years in terms of distortion and the like. Regarding imaging lenses disclosed in Japanese Patent Nos. 3595897 and 3281583, entire lengths thereof are comparatively long, and there are further rooms for improvement in terms of a compact size. Regarding an imaging lens disclosed in Japanese Patent No. 4248956, the f-number thereof is 3.5 and is a little dark, and there is further room for improvement in terms of brightness.
In addition, an imaging lens disclosed in Japanese Patent No. 4245749 is known as an imaging lens preferably used for an imaging apparatus in which a solid-state image sensing device is used. However, a scanning lens thereof includes four lens groups having five lenses, and a lens configuration thereof is different from that of an imaging lens according to the present invention.